1. Field of the Invention
This invention relates to a frequency modulator comprising an adjusting circuit, by which frequency modulator an applied luminance signal of a color video signal that contains synchronization pulses is frequency modulated, while the chrominance signal of the color video signal is modulated on a carrier for which there is provided a reference frequency.
Such frequency modulators are installed in video recorders in which the luminance signal of a color video signal is converted into a frequency modulated signal by the frequency modulator and is recorded in that form. Usually the frequencies and the frequency deviation for this frequency modulation are exactly laid down in the specifications of the recording standard. For tapes recorded in this manner to be exchanged between various devices, these specifications should be exactly followed. The tolerances of these frequencies are so narrow that they cannot be satisfied in an integrated circuit without further measures being taken. For example, the VHS standard prescriber 3.8 MHz for the instantaneous frequency during the synchronization pulses and 4.8 MHz for white picture information.
2. Description of the Related Art
Therefore, in prior-art circuit concepts two adjusting points are provided, that is to say, for separately adjusting the frequency for the synchronization pulses and for the white picture information. This adjustment may optionally be effected on the chip during the manufacture of the IC. However, this is detrimental in that the circuit cannot be adjusted in its future environment within the apparatus with the temperature, voltage and further influences prevailing there and, therefore, in the end this is not very accurate.
U.S. Pat. No. 4,918,529 discloses an adjusting circuit for a modulator, for the operation of which adjusting circuit, a separate pulse is fed into the video signal, which pulse adopts certain voltage conditions on the basis of which the modulator is adjusted. The disadvantage of this circuit arrangement is mainly that an additional pulse is to be generated, fed and, thereafter, identified at the adjustment.